Patent GB-596649-A (GB)
Title: Improvements in or relating to radar systems
IPC: G01S13/53
Filed: 1944-12-06
Abstract: 596,649. Radiolocation ;valve circuits. ELLIOTT, W. S., BAILEY, A. E., and GAUNT, H. Dec. 6, 1944, No. 24449. [Class 40 (v)] In a pulse radiolocation system for the detection of moving objects, artificial echo or " strobe " signals are fed to the range indicating device when a Doppler beat-frequency is detected in the signals returned from an object of the same range as the strobe signals. The strobe signals may be arranged to perform an automatic search in range between predetermined limits, each element of range corresponding to the position of the strobe pulses, being examined for a sufficient time interval to enable several cycles of the lowest Doppler beat-frequency to be detected. In the radiolocation system of Fig. 1, the aerial 2 which has a beam width of 30 degrees is continuously rotated at 3 r.p.m. so that the beam width is scanned in approximately 1.5 secs. The range scan can thus occupy 1.5 sees. and with a strobe pulse of duration 10 microseconds corresponding to a range interval of 1 mile, the range from 0 to 15 miles only, is scanned as it is desired to observe each element for at least 0.1 sees. to detect a Doppler beat of about 30 c.p.s. which will be produced by a radial target velocity of 50 m.p.h. with the radio frequency of 212 mc/s. employed. The aerial 2 is coupled to the. transmitter 1 and two receivers 5 and 6 through a rotating joint 3 and switch unit 4. The output of receiver 5, which is suppressed for ranges from 0 to 15 miles by a pulse from the multivibrator 16 which is locked to the transmitter pulse recurrence over line 8, is fed directly to the Plan Position Indicator 7. To generate the Doppler beat frequencies in receiver 6, reference oscillations of true I.F. frequency, coherent in phase with the transmitted radio-frequency, are injected into the second detector. A 212 Mc/s. oscillator 11 is locked in phase with radio - frequency fed from the transmitter over buffer amplifier 10, the oscillator output being mixed at 12 with the 182 Mc/s. local oscillation from receiver 6. The I.F. resultant is fed through buffer amplifier 14 to the second detector of the receiver. If the output of oscillator 11 had been fed directly to receiver instead of feeding the I.F. to the second detector, overloading would have resulted. The strobe pulses which open the normally blocked receiver 6 and are variably timed are produced by the saw-tooth generator 18 locked to the pulse recurrence frequency and controlled by the slow speed triangular wave generator 17. The strobe pulses are also fed via a gate valve 24 to the P.P.I. 7, the gate valve being opened by a pulse generated when the presence of a Doppler beat is detected in the output of receiver 6 by the circuit comprising pulse lengthener 20 Doppler beat frequency filter 21, amplifier 22 and integrator 23. Strobe generating circuits, Fig. 5. Valves V 17, V18 form a' multivibrator with the common cathode resistance R15, the grid of V17 7 being connected to a point at, say, 250 v. on potential divider R12 over high resistance R13, and the lower voltage limit of the cathode being set by the connection of the grid of cathode follower V25 to a point at, say, 150 v. on potential divider R11. Due to the anode load R10, V18 passes less current than V17 so that while V17 is conducting a positive voltage is impressed on the grid of valve V20 through the long-timeconstant circuit C5, R16. The anode voltage of V20 is thus low and the grid voltage of valve V19, which is connected in a Miller integrator circuit by means of condenser C4 and resistance R14, will tend to fall and as a result the anode voltage of V19 rises linearly, until due to the connection to the grid of valve V18 that valve is rendered conducting. The multivibrator then reverses and negative voltage is applied to the grid of valve V20, cutting it off. The grid of valve V19 then tends to follow the rise in anode voltage of valve V20 with the result that the anode voltage of valve V19 falls linearly forming the other flank of the triangular wave, until the grid voltage of valve V18 falls'below the cathode voltage set by the cathode' follower V25, causing the multivibrator to revert to its original state and recommence the cycle. The slope of the triangular wave, and thus its period which is 3 seconds, is controlled by the resistance R14 and the limits of voltage excursion are set by the potential dividers R11, R12. Valve V21 is similarly connected as a Miller integrator; the suppressor grid bias being such that all current normally nows to the screen. Positive recurrence frequency lock pulses applied to the suppressor permit anode current to flow, and due to the feed-back through condenser C7 and the connection of the grid resistance R21 to a positive source, the anode voltage falls linearly until anode current increase ceases, the screen current increases and the anode is cut off by the suppressor grid bias. The duration of the negative-going saw-tooth depends upon the voltage through which the anode of V21 falls and this is set by connecting the anode through diode V22 to the anode of valve V19. The duration of the saw-tooth waves and thus the width of the negative pulses from the screen of valve V21 thus vary in accordance with the slow speed triangular wave, the minimum duration being set by the controls R20, R22 which adjust the lower limit of grid voltage fall for valve V21. The square wave at the screen of valve V21 is differentiated by circuit C8, R23 and the negative pulse of variable timing, corresponding to the back edge of the square wave cuts off valve V23 to produce the positive strobe pulse which is fed out via the cathode follower V24. Manual control of the strobe position may be obtained by means of potential divider R24 if switch 45 is changed over or alternatively the slow speed waveform may be obtained from a potential divider driven by the aerial scanning mechanism. Detection of Doppler beat.-The signal output of receiver 6 due to a moving target comprises pulses amplitude modulated at the Doppler shift frequency and this wave is fed to a pulselengthening circuit 20, comprising a valve, Fig. 3 (not shown), with a long-time-constant resistance-condenser combination in its cathode circuit, so that the energy content of the wave is increased. The filter 21 has a pass-band of 30 to 300 c.p.s. and extracts the Doppler beatfrequency which is fed to the integrator circuit, Fig. 4. Valves V8, V9 form a multivibrator with a common cathode resistance formed by the constant current pentode V10 and the grid voltages are such that valve V9 is normally conducting. If a Doppler beat-frequency is applied to the grid of the valve V7, the multivibrator is caused to reverse when the rising anode voltage of V7 exceeds the grid voltage derived from potential divider R4, the following fall of voltage causing the circuit to revert to its quiescent state when the grid voltage of valve V8 falls below the steady voltage derived from potential divider R4. The amplitude of the negative-going square wave at the anode of valve V8 is controlled by the bias applied to valve V10 from potential divider R5, so that the position and height of the " slice " of the input wave which operates the circuit is controlled by R4 and R5. Each negative pulse at the anode of valve V8 charges up condenser C2 through diode V11, the negative charge increasing in steps with each cycle of the Doppler beat-frequency until the gas discharge tube V12 fires and produces at its anode a short negative pulse which charges up condenser C3 through diode V13. Resistance R8 is high so that an extended negative voltage is applied to the grid of valve V14 which is cut off for about 1 /10th second following the triggering of V12. The positive square wave at the anode of valve V14 is fed to the suppressor grid of the gate valve V16, a diode V15 preventing the suppressor assuming a positive bias, while the strobe pulses are fed to the grid from terminal 41. Strobe pulses whose amplitude may be adjusted by the screen voltage control R9 are thus obtained from the output terminal 39 and fed to the P.P.I. only when a sufficient number of cycles of input voltage have been obtained to operate the integration circuit. Specifications 596,650 and 596,657 are referred to.